components suitable for filters

[andy257]

Hi All,

I have been told that some types of capactitors and certain types of resistors are not suitable for filter design. Can anyone elaborate on what types are suitable for filters (active).

thanks

Andy

 

[chemelec]

Hi All,

I have been told that some types of capactitors and certain types of resistors are not suitable for filter design. Can anyone elaborate on what types are suitable for filters (active).

thanks

Andy

Depends a lot on How Good of a Filter you want?
For just an average quality, active audio filter, it won't make too much difference.
Also higher frequencys become more critical on capacitors.

Most resistors are OK, but 1% values may be required.

Capacitors: For better quality, definately non-polarized, low leakage types. Mylar are usually OK.
(Polystyrene and NPO types are High Quality)

 

[Optikon]

Hi All,

I have been told that some types of capactitors and certain types of resistors are not suitable for filter design. Can anyone elaborate on what types are suitable for filters (active).

thanks

Andy

The main thing to watch out for is when the resistor or capacitor do not represent the ideal component that you probably assumed when you did your design. Resistors drift with temperature & time, capacitors drift with temperature and time also exhibit effects like dielectric absorption and so on.. if your design requires that a resistor not change it's value over temperature and time then you need to pick a suitable resistor.. that how you have to think about the problem. Don't go looking for rules of thumb and everyones opinions about certain types. Use facts provided by the datasheets and analyze your design deeper to understand the impact of non-ideal components.

 

[dknguyen]

ELectrolytic capacitors are probably the worst but come in the largest values and are the cheapest. There is tantalum too which is smaller (and some say less reliable) but better frequency characteristics. Both are polarized and cannot be used where the polarity may be reversed.

THen you have ceramics (non-polarized so can be used in bipolar applications) which come in different kinds of varying size ranges, accuracies, and stabilities (with DC bias and temperature). C0G is the most common one for this but has the smallest sizes. X7R is the next one that is kind of stable (15% or so).

Beyond there you have the realy good, but tiny capacitors like polyester film and mica...costs an arm and a leg sometimes.

Not much to say about resistors other than wirewounds don't work well at high frequencies due to inductance. Metal film is a nice balance between noise and cost. Carbon is the cheapest but I think can be the noisiest and least stable/accurate of the common types.

 

[Dean Huster]

Avoid ceramic capacitors at all costs. Most have TERRIBLE temperature coefficients and should never be used in frequency-dependent circuits such as filters, oscillators and timers. For audio frequency work, polyester (Mylar), polystyrene, poly .... well, most anything starting with "poly" .... is better. For higher-frequency stuff, NPO ceramics (the only temperature-stable ceramics) are OK as are mica caps. For really low stuff, use tantalum caps rather than aluminum electrolytics as they're more temperature stable, have less leakage, lower ESR and more-accurate values. Most aluminum electrolytics have tolerances of -20%/+80% which is 'way too wild for frequency-dependent circuits. And large electrolytics simply will not allow a 555 timer circuit to work at all, especially if the timing resistors are large.

Although the resistors aren't quite as critical, carbon composition (especially) and carbon film resistors are not my choice. Metal film types are great .... more accurate and a lot more stable. They're cheap now compared to what they were in 1965 (at $1.50 each in 1965, they were more expensive that high-end transistors). Today, you can get a package of 200 for under $4.00. Thank China, I guess.

Dean